Conventionally, a wire bonding method applying soldering has been widely used in mounting a semiconductor device onto the input-output terminal electrodes of a circuit board. However, with the recent size minimization of semiconductor device packages and increase in the number of connecting terminals, gaps between connecting terminals are becoming narrow, so that it is increasingly difficult to mount using the conventional soldering technique.
Thus, methods have been proposed recently in which the mounting area is minimized by directly mounting a semiconductor device such as an integrated circuit chip onto the input-output terminal electrodes of a circuit board.
Among these methods, a flip chip mounting method--a method of mounting a semiconductor device face down onto a circuit board--can electrically connect the semiconductor device and the circuit board at the same time, and has excellent mechanical strength after the bonding process.
For example, a mounting method applying a soldering plating method is described in IC KA JISSOU GIJYUTSU (edited by Japan Microelectronics Association and Published by Institute for Industrial Research on Jan. 15, 1980), and the method is explained below.
FIG. 7(a) is a cross-sectional view of a solder bump in a conventional semiconductor device. FIG. 7(b) is a cross-sectional view of a conventional semiconductor unit. In connecting an electrode pad 113 of a semiconductor device (IC substrate) 116 to an input-output terminal electrode 118 of a circuit board 119, an electrical contact 110 (hereinafter, referred to as "solder bump") made of solder is formed by a plating method on a diffusion-preventing metal film 111 after forming an adhering metal film 112 and diffusion-preventing metal film 111 on electrode pad 113 by a deposition method (FIG. 7(a)). Then, as shown in FIG. 7(b), the IC chip prepared as mentioned above (FIG. 7(a)) is positioned so as to place solder bump 110 face down onto input-output terminal electrode 118 of circuit board 119. By heating the mounted body (semiconductor unit) of semiconductor device (IC substrate) 116 at a high temperature, solder bump 110 is melted and adhered to input-output terminal electrode 118 of circuit board 119.
A semiconductor unit as shown in FIG. 8 also has been proposed recently. FIG. 8 is a cross-sectional view of a conventional semiconductor unit in which a conductive adhesive is applied. As shown in the figure, an electrical contact (Au bump) 120 is formed onto an electrode pad 123 of a semiconductor device (IC substrate) 126 by a wire bonding method or a plating method. Through conductive adhesive (bonding layer) 125, Au bump 120 is connected to an input-output terminal electrode 128 of a circuit board 129. In this semiconductor unit, conductive adhesive 125 is transferred onto Au bump 120 of semiconductor device 126, and Au bump 120 is then positioned so as to be placed onto input-output terminal electrode 128 of circuit board 129. Conductive adhesive 125 is cured, thus establishing an electrical connection.
Furthermore, a semiconductor unit is proposed which is underfilled with under-fill resin so as to enhance connection and bonding between a semiconductor device and a circuit board. In this semiconductor unit, the process of underfilling the under-fill resin and a curing process have to be carried out.
The conventional semiconductor devices and mounted bodies (semiconductor units) mentioned above, however, have the following problems.
Although the terminal electrode of the circuit board usually has a surface made of Au, Au has poor reactivity, so that the conductive adhesive and the terminal electrode are unlikely to adhere to each other. In addition to poor adherence, contact resistance at the boundary between the conductive adhesive and the terminal electrode is high. Contact resistance also is high at the boundary between the protruding electrode of the semiconductor device and the conductive adhesive. Therefore, the connection and bonding between the semiconductor device and the cirucuit board are not reliable.
Particularly, in carrying out a thermal test such as the temperature cycle test, the adhesive strength is weakened considerably by the stress of thermal expansion and also by moisture absorption since there are differences in thermal expansion coefficients between the semiconductor device, circuit board and under-fill resin, thus forming cracks and peeling at bulk sections. As a result, the boundary connection and bonding become weak, and the resistance of the electrical contact (Au bump) increases.